In this post, we will put into words on how to fix WordPress RSS feed errors. Commonly, poor formatting is cause of the WordPress RSS feed errors. The first option is fixing the feed error manually. You need to edit your functions file if you recently added a code snippet to your theme’s functions.php file. Just make sure that there is no extra space or line breaks after it in closing php tag at the end of your functions file. Closing PHP tag is not compulsory at the end of the file but it would be the best if you remove the closing php tag. If this will not fix the problem, the second option is fixing these errors using plugin. You need to install and activate first the Fix My Feed RSS Repair plugin. On activation, simply go to Tools > RSS Feed Fix page. Lastly, click on the Fix feed button and finish.
For further information check the full article at How to Fix WordPress RSS Feed Errors.